Double stacked projection

ABSTRACT

A method for producing a first output image and a second output image for being projected by a first projector and a second projector, respectively, is disclosed. The method comprises: providing a source image comprising a plurality of pixels, each pixel having a source value, providing an inverted threshold value for each pixel of the plurality of pixels, and generating thereof a temporary image comprising a temporary value for each pixel of the plurality of pixels. The method further comprises: generating the first output image comprising a first output value for each pixel of the plurality of pixels, the first output value being generated from the temporary value and the source value for each pixel, and generating the second output image comprising a second output value for each pixel of the plurality of pixels, the second output value being generated from the temporary value.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a U.S. national phase under 35 U.S.C. 371 of InternationalPatent Application No. PCT/EP2011/056173, entitled “Double StackedProjection” and filed Apr. 18, 2011, which claims benefit of priorityunder PCT Article 8 of Danish Application No. PA 201000320 filed Apr.18, 2010, Danish Application No. PA 201000321 filed Apr. 18, 2010, andU.S. Provisional Application No. 61/356,980 filed Jun. 21, 2010. Eachand every application is incorporated herein by reference in itsentirety.

BACKGROUND OF THE INVENTION

Modern super high resolution 4K digital cinema projectors designed fornormal sized cinema screens have a resolution ideal also for very largescreens but lack the brightness needed for these. Double stackingprojectors are an effective way of increasing brightness, buttraditional double stacking is difficult at such high resolutionsbecause the tolerance in the alignment of projected images becomes verysmall and is hard to meet during presentations due to thermal inducedmovements in the mechanical and optical parts and vibrations from theaudio system. In other applications like temporary projection set-ups,home cinemas etc., alignment of double stacked projectors may bedifficult to maintain even when working at much lower resolutions.

“Double stacking” of projectors, i.e. overlaying the images of twoprojectors projecting the same image, is a well known way to increasebrightness. However, it is also well known that traditional doublestacking requires high maintenance of the alignment of the projectors tomaintain image quality.

In 4K projection, traditional double stacking is not considered anoption, because it would be impossible to keep the sharpness and detailon par with that of a single 4K projector throughout a presentation.This is unfortunate for giant screen theatres, because while 4Kprojectors lend themselves well to giant screens in terms of resolution,available projectors generally do not have enough light for giantscreens, so stacking would seem desirable to double the light output.

OBJECT OF THE INVENTION

An object of the invention is to present a double stacking system thatovercomes the above mentioned difficulty and presents other advantages.Exemplary applications may be giant screen cinemas, simulators,conference presentations, staging, exhibits, outdoor projection,traditional cinemas, home cinemas, and other applications wherebrightness of a projected image is a consideration.

An object of the present invention is also to present a novel imageprocessing system for double stacked projector configurations thatovercomes the above mentioned maintenance difficulties and provides fora high quality, low-maintenance double stacking system even for 4Kprojection.

SUMMARY OF THE INVENTION

An image processing circuit comprising thresholding limiters andconstrained smoothing filters splits a source image into two images,which, when projected overlaid on a projection surface by a pair ofdouble-stacked projectors, together form an image essentially identicalto the source image, but where one image has significantly less highfrequency components. The invention presents advantages over traditionaldouble stacking in aspects of projector alignment, content copyprotection, banding artefacts and equipment costs.

GENERAL DESCRIPTION

The above objects are according to a first aspect of the presentinvention met by a method for producing a first output image and asecond output image for being projected by a first projector and asecond projector, respectively, the method comprising:

(a) providing a source image comprising a plurality of pixels, eachpixel having an source value,

(b) providing a threshold value for each pixel of the plurality ofpixels, in a first alternative

(d) generating a temporary image comprising a temporary value for eachpixel of the plurality of pixels, the temporary value being generated ina process equivalent to: (i.i) determining a first maximum value as themaximum of the source value and its corresponding threshold value foreach pixel, (i.ii) determining an intermediate value by subtracting thecorresponding threshold value from the first maximum value for eachpixel, (i.iii) generating the temporary value from the intermediatevalue for each pixel;or in a second alternative(c) providing an inverted threshold value for each pixel of theplurality of pixels, each inverted threshold value being an inversion ofits corresponding threshold value,(d) generating a temporary image comprising a temporary value for eachpixel of the plurality of pixels, the temporary value being generated ina process equivalent to: (i.i) determining an intermediate value as theminimum of the source value and its corresponding inverted thresholdvalue for each pixel, (i.ii) generating the temporary value from theintermediate value for each pixel;or in a third alternative(c) providing an inverted threshold value for each pixel of theplurality of pixels, each inverted threshold value being an inversion ofits corresponding threshold value,(d) generating a temporary image comprising a temporary value for eachpixel of the plurality of pixels, the temporary value being generated ina process equivalent to: (i.i) determining a first maximum value as themaximum of the source value and its corresponding threshold value foreach pixel, (i.ii) determining a first difference value by subtractingthe corresponding threshold value from the first maximum value for eachpixel, (i.iii) determining a first minimum value as the minimum of thesource value and its corresponding inverted threshold value for eachpixel, (i.iv) determining an intermediate value as the minimum of thefirst difference value and the first minimum value for each pixel, (i.v)generating the temporary value from the intermediate value for eachpixel;or in a fourth alternative(c) providing an inverted threshold value for each pixel of theplurality of pixels, each inverted threshold value being an inversion ofits corresponding threshold value,(d) generating a temporary image comprising a temporary value for eachpixel of the plurality of pixels, the temporary value being generated ina process equivalent to: (i.i) determining a first maximum value as themaximum of the source value and its corresponding threshold value foreach pixel, (i.ii) determining a first difference value by subtractingthe corresponding threshold value from the first maximum value for eachpixel, (i.iii) determining a first minimum value as the minimum of thesource value and its corresponding inverted threshold value for eachpixel, (i.iv) determining an intermediate value from a first range ofvalues comprising values between the first difference value and thefirst minimum value for each pixel, (i.v) generating the temporary valuefrom the intermediate value for each pixel; and in all alternatives(e) generating the first output image comprising a first output valuefor each pixel of the plurality of pixels, the first output value beinggenerated from the temporary value and the source value for each pixel,and(f) generating the second output image comprising a second output valuefor each pixel of the plurality of pixels, the second output value beinggenerated from the temporary value.

The method according to the first aspect of the present invention mayfurther comprise in the first alternative:

(c) providing an inverted threshold values for each pixel of theplurality of pixels, each inverted threshold value being an inversion ofits corresponding threshold value.

The threshold value for each pixel of said plurality of pixels may belimited to be within an interval having a maximum threshold value and aminimum threshold value for each pixel. Each inverted threshold valuebeing an inversion of its corresponding threshold value may beunderstood as equivalent to the inverted threshold value being equal toor approximately equal to the maximum threshold value minus thethreshold value for each pixel.

The process of generating the temporary value may further comprise inall alternatives: (i.vi) smoothing the intermediate value for eachpixel; and in the third and fourth alternatives: (i.vi) smoothing thefirst difference value and/or the first minimum value.

Smoothing the intermediate value of a pixel is here understood toinvolve the intermediate value of at least one other pixel, for examplea neighbouring pixel. Smoothing the first difference value of a pixel ishere understood to involve the first difference value of at least oneother pixel, for example a neighbouring pixel. Smoothing the firstminimum value of a pixel is here understood to involve the first minimumvalue of at least one other pixel, for example a neighbouring pixel. Thesmoothing may comprise a spline filter, a membrane filter, and/or anenvelope filter.

The smoothing may be adapted for limiting the intermediate value to avalue from the first range of values subsequent to the smoothing. Thesmoothing may comprise a first dilation operation comprising a firstdilation radius. The first dilation radius may be 4 pixels, orapproximately 0.3% of the width of the temporary image. The smoothingmay comprise a first blur operation. The first dilation operation may beperformed prior to the first blur operation. The first blur operationmay comprise a first blur radius approximately equal to or smaller thanthe first dilation radius. The first blur operation may comprise a firstGaussian blur operation. The first Gaussian blur operation may have astandard deviation approximately equal to a third of the first blurradius, or approximately equal to or smaller than 4/3 pixels, orapproximately 0.1% of the width of the temporary image. The first bluroperation may comprise a first mean filtering operation.

The process generating the temporary value may further comprise: (i.vii)determining a second minimum value as the minimum of the intermediatevalue and the inverted threshold value for each pixel, (i.viii)generating a second smoothed value by smoothing the second minimum valuefor each pixel, and (i.ix) generating the temporary value from thesecond smoothed value for each pixel.

Smoothing the second minimum value of a pixel is here understood toinvolve the second minimum value of at least one other pixel, forexample a neighbouring pixel. The smoothing of the second minimum valuemay comprise a spline filter, a membrane filter, and/or an envelopefilter.

The smoothing of the second minimum value may comprise a second dilationoperation comprising a second dilation radius. The second dilationradius may be 2 pixels, or approximately 0.17% of the width of thetemporary image. The second dilation radius may be variable. The seconddilation radius may be variable in a second range of values includingzero. The smoothing of the second minimum value may comprise a secondblur operation. The second dilation operation may be performed prior tothe second blur operation. The second blur operation may comprise asecond blur radius approximately equal to or smaller than the seconddilation radius. The second blur radius may be variable. The second blurradius may be variable in a third range of values including zero. Thesecond blur radius and the second dilation radius may be coupled suchthat one changes as a function of the other.

The second blur operation may comprise a second Gaussian blur operation.The second Gaussian blur operation may have a standard deviationapproximately equal to a third of the first blur radius, orapproximately equal to or smaller than ⅔ pixels, or approximately 0.055%of the width of the temporary image.

The second blur operation may comprise a second mean filteringoperation.

Providing the source image may comprise: (ii.i) providing a gammaencoded source image encoded by a first gamma encoding, (ii.ii)generating a gamma decoded source image by performing a first gammadecoding of the gamma encoded source image, the gamma decodingcorresponding to the first gamma encoding, and (ii.iii) outputting thegamma decoded source image as the source image.

The method according to the first aspect of the present invention mayfurther comprise:

(g) performing a second gamma encoding of the first output image, thesecond gamma encoding corresponding to a second gamma decoding of thefirst projector.

The method according to the first aspect of the present invention mayfurther comprise:

(h) performing a third gamma encoding of the second output image, thethird gamma encoding corresponding to a third gamma decoding of thesecond projector.

The process of generating the temporary value may further comprise inall alternatives: (i.x) performing a first colour correction of theintermediate value for each pixel, and in the third and fourthalternatives: (i.x) performing a first colour correction of theintermediate and/or the first difference value for each pixel.

In all alternatives the first colour correction may be adapted forcorrecting the intermediate value to obtain approximately the same firsthue as the corresponding source value and in the third and fourthalternative the first colour correction being adapted for correcting thefirst difference value and/or the intermediate value to obtainapproximately the same first hue as the corresponding source value. Thefirst colour correction may comprise a process equivalent to: (iii.i)calculating a constant K for each pixel, K being equal to the maximum ofR11/R6, G11/G6, and B11/B6; R6, G6, and B6 are the pixel colours of thesource image; and R11, G11, and B11 are the pixel colour valuessubsequent to determining the first intermediate value for each pixel,(iii.ii) correcting the intermediate value by replacing it with thesource value multiplied with the constant K for each pixel.

The method according to the first aspect of the present invention mayfurther comprise: (i) lowering the spatial resolution of the secondoutput image and/or performing a blur operation on the second outputimage. The method according to the first aspect of the present inventionmay further comprise: (j) encrypting the first output image. The methodaccording to the first aspect of the present invention may furthercomprise: (k) recording the first output image on a first recordingmedium. The method according to the first aspect of the presentinvention may further comprise: (l) extracting the first output imagefrom the first recording medium. The method according to the firstaspect of the present invention may further comprise: (m) recording thesecond output image on a second recording medium. The method accordingto the first aspect of the present invention may further comprise: (n)extracting the second output image from the second recording medium.

The method according to the first aspect of the present invention mayfurther comprise: (o) performing a geometric correction of the secondoutput image, the geometric correction being adapted for aligning animage projected by the second projector with an image projected by thefirst projector.

The process of generating the temporary value may further comprise:(i.xi) performing an erosion operation, preferably a grey scale erosionoperation having a radius a half pixel, a full pixel, 0.04% of the widthof temporary image, or 0.08% of the width of temporary image, on theintermediate value for each pixel of the plurality of pixels.

In the fourth alternative, the source value may be excluded from thefirst range of values for each pixel. In the fourth alternative, thefirst range of values may further comprise the first difference valueand the first minimum value.

The first output value may be generated for each pixel in a processequivalent to: (iv.i) determining a second difference value bysubtracting the temporary value from the source value for each pixel,and (iv.ii) generating the first output value from the second differencevalue.

The first output value may be generated for each pixel in a processequivalent to: (iv.i) determining a second difference value bysubtracting the temporary value from the source value for each pixel,(iv.ii) generating a first ratio by dividing the second difference valueby the threshold value for each pixel, and (iv.iii) generating the firstoutput value from the first ratio for each pixel.

The second output value may further be generated from the invertedthreshold value. The second output value may be generated for each pixelin a process equivalent to: (v.i) generating a second ratio by dividingthe temporary value by the inverted threshold value for each pixel, and(v.ii) generating the second output value from the second ratio for eachpixel.

The threshold value for each pixel of the plurality of pixels mayrepresent the fraction of the total illumination intensity which thefirst projector contributes to at the corresponding position on theprojection surface in a projection of a uniform and maximum intensityimage from the first projector and the second projector, or in aprojection of a uniform and maximum intensity image from each the firstprojector and the second projector, or in a projection of a uniform andmaximum intensity image from the first projector, or in a projection ofa uniform and maximum intensity image from the second projector.

The threshold value for each pixel of the plurality of pixels may bederived by dividing the total illumination intensity, which the firstprojector contributes to at the corresponding position on the projectionsurface by the combined total illumination intensity from each of thefirst projector and the second projector at the corresponding positionin a projection of a uniform and maximum intensity image.

The method according to the first aspect of the present invention mayfurther comprise:

(p) adjusting the temporary image to include an alignment pattern.

The method according to the first aspect of the present invention mayfurther comprise:

(q) providing the alignment pattern,

(r) adjusting the temporary image by adding the alignment pattern to thetemporary image,

(s) adjusting the temporary image by a process equivalent to: (vi.i)determining a fourth minimum value as the minimum of the temporary valueand its corresponding source value for each pixel, and (vi.ii) adjustingthe temporary value to the fourth minimum value for each pixel.

The alignment pattern may comprise a grid, a mesh, a barcode, and/or asemacode, and alternatively or additionally the alignment patterncomprising a regular pattern of elements, and/or an irregular pattern ofelements, and alternatively or additionally the alignment patterncomprising a regular pattern of dots and/or cross hairs, and/or anirregular pattern of elements of dots and/or cross hairs.

The above objects are according to a second aspect of the presentinvention met by a method for double stacking a first output image and asecond output image on a projection surface by a first projector and asecond projector, the method comprising:

(aa) positioning and orienting the first projector and the secondprojector for overlaying the first output image and the second outputimage on the projection surface,

(ab) producing the first output image and the second output image by themethod according to the first aspect of the present invention,

(ac) supplying the first output image and the second output image to thefirst projector and the second projector, respectively, and

(ad) projecting the first output image and the second output image bythe first projector and the second projector, respectively.

The first projector and the second projector may generate a superimposedimage on the projection surface. The method according to the secondaspect of the present invention may further comprise:

(ae) recording a first captured image of the superimposed image,

(af) determining a first contribution of the first projector to thefirst captured image,

(ag) generating a first feedback image from the first contribution,

(ah) generating a first set of misalignment vectors from the firstfeedback image and the first output image by a feature tracking and/orfeature matching,

(ai) generating a first warped image of the first captured image by afirst warping comprising the first set of misalignment vectors,

(aj) generating a second feedback image by subtracting the first outputimage from the first warped image,

(ak) generating a second set of misalignment vectors from the secondfeedback image and the second output image by a feature tracking and/orfeature matching,

(al) generating a third set of misalignment vectors from the first setof misalignment vectors and the second set of misalignment vectors, and

(am) deriving a first geometric correction of the first output imageand/or the second output image from the third set of misalignmentvectors.

Determining the first contribution of the first projector may comprise ahigh pass filtering of the first captured image.

The above objects are according to a third aspect of the presentinvention met by a method for deriving a correction of a double stackingof a first output image and a second output image on a projectionsurface by a first projector and a second projector, the methodcomprising:

(ba) positioning and orienting the first projector and the secondprojector for overlaying the first output image and the second outputimage on the projection surface,

(bb) producing a first output for a first source image, the first outputcomprising the first output image and the second output image producedby the method according to an example of the first aspect of the presentinvention including an alignment pattern for the first source image,(bc) supplying the first output image and the second output image of thefirst output to the first projector and the second projector,respectively, and(bd) projecting the first output image and the second output image ofthe first output by the first projector and the second projector,respectively, on the projection surface,(be) recording a first captured image comprising the first output imageand the second output image of the first output projected on theprojection surface,(bf) detecting a contribution of the misalignment pattern of the firstoutput in the first captured image(bg) deriving a geometric correction for the second output image fromthe detected contribution of the misalignment pattern of the firstoutput.

The method according the second aspect of the present invention mayfurther comprise:

(bh) producing a second output for a second source image for beingdisplayed subsequent to the first source image, the second outputcomprising the first output image and the second output image producedby the method according to an example of the first aspect of the presentinvention including an alignment pattern for the second source image,(bi) supplying the second output image and the second output image ofthe second output to the first projector and the second projector,respectively, and(bj) projecting the second output image and the second output image ofthe second output by the first projector and the second projector,respectively, on the projection surface,(bk) recording a second captured image comprising the first output imageand the second output image of the second output projected on theprojection surface,(bl) detecting a contribution of the misalignment pattern of the secondoutput in the second captured image,(bm) deriving a geometric correction for the second output image fromthe detected contribution of the misalignment pattern of the secondoutput.

The method according the second aspect of the present invention mayfurther comprise:

(bh) producing a second output for a second source image for beingdisplayed subsequent to the first source image, the second outputcomprising the first output image and the second output image producedby the method according to an example of the first aspect of the presentinvention including an alignment pattern for the second source image,(bi) supplying the second output image and the second output image ofthe second output to the first projector and the second projector,respectively, and(bj) projecting the second output image and the second output image ofthe second output by the first projector and the second projector,respectively, on the projection surface,(bk) recording the first captured image comprising the first outputimage and the second output image of the second output projected on theprojection surface,(bl) detecting a contribution of the misalignment pattern of the firstoutput in the first captured image further comprising detecting acontribution of the misalignment pattern of the second output in thefirst captured image,(bm) deriving a geometric correction for the second output image fromthe detected contribution of the misalignment pattern of the firstoutput and the second output.

Detecting a contribution of the misalignment pattern of the first outputin the first captured image and detecting the contribution of themisalignment pattern of the second output in the second captured imagemay further comprise a time averaging of the first captured image andthe second captured image. Detecting of a contribution of themisalignment pattern of the first output and the second output maycomprise high pass filtering.

The misalignment pattern of the first output and the misalignmentpattern of the second output may be the same. The misalignment patternof the first output and the misalignment pattern of the second outputmay be different. The misalignment pattern of the second output may begenerated from the misalignment pattern of the first output. Themisalignment pattern of the second output and the misalignment patternof the first output may be generated by a cyclic function, the cyclicfunction being periodic as a function of time.

The above objects are according to a fourth aspect of the presentinvention met by a method for producing a first output image and asecond output image of a first colour for being projected by a firstprojector and a second projector, and for producing a first output imageand a second output image of a second colour for being projected by thefirst projector and the second projector, the method comprising:

(ca) producing the first output image and the second output image of thefirst colour by the method according to the first aspect of the presentinvention, and

(cb) producing the first output image and the second output image of thesecond colour by the method according to the first aspect of the presentinvention.

The above objects are according to a fifth aspect of the presentinvention met by a method for producing a first output image and asecond output image of a first colour for being projected by a firstprojector and a second projector for projecting the first colour, andfor producing and a first output image and a second output image of asecond colour for being projected by a first projector and a secondprojector for projecting the second colour, the method comprising:

(ca) producing the first output image and the second output image of thefirst colour by the method according to the first aspect of the presentinvention, and

(cb) producing the first output image and the second output image of thesecond colour by the method according to an example of the first aspectof the present invention including an alignment pattern.

The first colour and the second colour may represent the left and rightcolours of stereoscopic image. The first colour and the second colourmay represent two colours of a colour model, for example the RGB colourmodel.

In the fourth and fifth aspects of the present invention, the producingof the first output image and the second output image of the firstcolour may be performed by the method according to an example of thefirst aspect of the present invention including an alignment pattern.The first colour may represent shorter light wavelengths than the secondcolour. The first colour may represent blue and the second colour mayrepresent green, yellow, or red.

The producing of the first output image and the second output image ofthe second colour may be performed by the method according to an exampleof the first aspect of the present invention including an alignmentpattern the alignment pattern in producing the first output image andthe second output image of the first colour and the alignment pattern inproducing the first output image and the second output image of thesecond colour may have the same or approximately the same shape. Thealignment pattern in producing the first output image and the secondoutput image of the first colour and the alignment pattern in producingthe first output image and the second output image of the second colourmay have the same or approximately the same dimensions.

The method according to the fourth aspect of the present invention mayfurther be adapted for producing a first output image and a secondoutput image of a third colour for being projected by the firstprojector and the second projector, the method may further comprise:

(cc) producing the first output image and the second output image of thethird colour by the method according to the first aspect of the presentinvention.

The first colour, the second colour, and the third colour may representthree colours of a colour model, for example the RGB colour model.

The method according to the fourth and fifth aspect of the presentinvention may further be adapted for producing a first output image anda second output image of a third colour for being projected by a firstprojector and a second projector for projecting the third colour, themethod may further comprise:

(cc) producing the first output image and the second output image of thethird colour by the method according to the first aspect of the presentinvention.

A first source value of a first pixel of the source image may representthe first colour, a second source value of a second pixel of the sourceimage may represent the second colour, and a third source value of athird pixel of the source image may represent the third colour, thecolours of the first, second and third pixels may define a second hue; afirst intermediate value may be the intermediate value of the firstpixel, a second intermediate value may be the intermediate value of thesecond pixel, and a third intermediate value may be the intermediatevalue of the third pixel defining a third hue, the method may furthercomprise:

(cd) subjecting the first, second, and third intermediate values to acolour adjustment.

The colour adjustment may be adapted for adjusting the first, second,and third intermediate values to define the third hue being equal to orapproximately equal to the second hue. The colour adjustment may beequivalent to: (vii.i) calculating a first fraction as the firstintermediate value divided by the first source value, (vii.ii)calculating a second fraction as the second intermediate value dividedby the second source value, (vii.iii) calculating a third fraction asthe third intermediate value divided by the third source value, (vii.iv)calculating a second maximum value as the maximum of the first, second,and third fractions, (vii.v) replacing the first intermediate value bythe first source value multiplied by the second maximum value, (vii.vi)replacing the second intermediate value by the second source valuemultiplied by the second maximum value, and (vii.vii) replacing thethird intermediate value by the third source value multiplied by thesecond maximum value.

The above objects are according to a sixth aspect of the presentinvention met by a system for producing a first output image and asecond output image for being projected by a first projector and asecond projector, respectively, the system comprising a computer and/orone or more circuits for performing the method according to the firstaspect of the present invention. The system according to the sixthaspect of the present invention may further comprise an image source forproviding the source image according to the first aspect of the presentinvention.

The above objects are according to a seventh aspect of the presentinvention met by a system for double stacking a first output image and asecond output image, the system comprising a first projector, a secondprojector, and a computer and/or one or more circuits for performing themethod according to the second aspect of the present invention. Thesystem according to the seventh aspect of the present invention mayfurther comprise an image source for providing the source imageaccording to the second aspect of the present invention. The systemaccording to the seventh aspect of the present invention may furthercomprise a camera for recording the first captured image of thesuperimposed image the second aspect of the present invention.

The above objects are according to an eighth aspect of the presentinvention met by a system for deriving a correction of a double stackingof a first output image and a second output image, the system comprisinga first projector, a second projector, and a computer and/or one or morecircuits for performing the method according to the third aspect of thepresent invention, the system further comprising a camera for recordingthe second captured image of the superimposed image.

The above objects are according to an ninth aspect of the presentinvention met by a system for producing a first output image and asecond output image of a first colour for being projected by a firstprojector and a second projector and a first output image and a secondoutput image of a second colour for being projected by the firstprojector and the second projector, the system comprising a computerand/or one or more circuits for performing the method according to thefifth and/or the sixth aspect of the present invention.

The above objects are according to a tenth aspect of the presentinvention met by a system for producing a first output image and asecond output image of a first colour for being projected by a firstprojector and a second projector for projecting the first colour and afirst output image and a second output image of a second colour forbeing projected by a first projector and a second projector forprojecting the second colour, the system comprising a computer and/orone or more circuits for performing the method according to the fifthaspect of the present invention.

The above objects are according to an eleventh aspect of the presentinvention met by a projection system comprising a first projector and asecond projector, the first projector comprising: a first lamp, a firstintegrating rod having an input end and an output end, the firstintegrating rod being configured for receiving light from the first lampthrough the input end and generate a uniform illumination at the outputend, a first projector filter configured to filter the uniformillumination at the output end of the integrating rod, a first spatiallight modulator chip, a first illumination system for imaging the firstprojector filter on the light modulator chip, a first exit pupil throughwhich light from the a first spatial light modulator chip exits thefirst projector; the second projector comprising: a second integratingrod having an input end and an output end, the second integrating rodbeing configured for receiving light from the second lamp through theinput end and generate a uniform illumination at the output end, asecond projector filter configured to filter the uniform illumination atthe output end of the integrating rod, a second spatial light modulatorchip, a second illumination system for imaging the second projectorfilter on the light modulator chip, a second exit pupil through whichlight from the a second spatial light modulator chip exits the secondprojector, the first projector filter being configured to wavelengthshift the light exiting through the first exit pupil, and the secondprojector filter being configured to wavelength shift the light exitingthe through the second exit pupil.

The first projector filter may define a first pass band and a firstguard band, and the second projector filter may define a second passband not overlapping the first pass band, and a second guard band mayoverlap the first guard band.

The first projector filter may define a first band stop and the firstprojector may further comprise: a first auxiliary filter configured tofilter the uniform illumination from the output end of the firstintegrating and defining a first pass band and a first guard band, andthe first band stop may match or approximately match the first guardband; and the second projector filter may define a second pass band notoverlapping the first pass band and a second guard band overlapping thefirst guard band.

The first projector filter may define a first band stop and the firstprojector may further comprise: a first auxiliary filter configured tofilter the uniform illumination from the output end of the firstintegrating and defining a first pass band and a first guard band, andthe first band stop may match or approximately match the first guardband, and the second projector filter may define a second band stop; andthe second projector may further comprise: a second auxiliary filterconfigured to filter the uniform illumination from the output end of thesecond integrating and defining a second pass band not overlapping thefirst pass band and a second guard band overlapping the first guardband, and the second band stop may match or approximately match thesecond guard band.

The second auxiliary filter may be flat and may have a second uniformthickness. The first auxiliary filter may be flat and may have a firstuniform thickness.

The first projector filter may define a first uniform thickness and/orthe second projector filter may define a second uniform thickness. Thefirst projector filter may have a first varying thickness and/or thesecond projector filter may have a second varying thickness. The firstprojector filter may define a first curvature and/or the secondprojector filter may define a second curvature. The first projectorfilter may define a first flat area in a first central portion of thefirst projector filter, and/or the second projector filter may define asecond flat area in a second central portion of the second projectorfilter. The first projector filter may define a first curved shape in afirst peripheral portion of the first projector filter, and/or thesecond projector filter may define a second curved shape in a secondperipheral portion of the second projector filter. The first projectorfilter may rest on a first transparent substrate, preferably a firstglass substrate, and/or the second projector filter may rest on a secondtransparent substrate, preferably a second glass substrate. The firstprojector filter may be dichroic, and/or the second projector filter maybe dichroic.

The first projector filter may be located at the output end of the firstintegrating rod and/or the second projector filter may be located at theoutput end of the second integrating rod. The first integrating rod maydefining a first aperture having a first width at the output end and thefirst projector filter may define a first spherical surface having afirst radius equal to or approximately equal to the first width, and/orthe second integrating rod may define a second aperture having a secondwidth at the output end and the second projector filter may define asecond spherical surface having a second radius equal to orapproximately equal to the second width.

The above objects are according to an twelfth aspect of the presentinvention met by a system for producing a series of three-dimensionalimages comprising: a computer and/or one or more circuits for producingleft output comprising first output images and second output images byrepeatedly applying the method according to the first aspect of thepresent invention, and the computer and/or the one or more circuitsfurther being adapted for producing right output comprising first outputimages and second output images by repeatedly applying the methodaccording to the first aspect of the present invention, the left outputrepresenting left perspective images of the series three-dimensionalimages and the right output representing corresponding right perspectiveimages of the series three-dimensional images; a projection screen; aleft perspective first projector coupled to the computer and/or one ormore circuits and configured for projecting the first output images ofthe left output on the projection screen; a right perspective firstprojector coupled to the computer and/or one or more circuits andconfigured for projecting the first output images of the right output onthe projection screen; and a left/right perspective second projectorcoupled to the computer and/or one or more circuits and configured foralternatingly projecting the second output images of the left output andthe second output images of the right output on the projection screen.

The above objects are according to an twelfth aspect of the presentinvention met by a system for producing a series of three-dimensionalimages comprising: a computer and/or one or more circuits for producingleft output comprising first output images and second output images byrepeatedly applying the method according to the first aspect of thepresent invention, and the computer and/or the one or more circuitsfurther being adapted for producing right output comprising first outputimages and second output images by repeatedly applying the methodaccording to the first aspect of the present invention, the left outputrepresenting left perspective images of the series three-dimensionalimages and the right output representing corresponding right perspectiveimages of the series three-dimensional images; a projection screen; aleft perspective first projector coupled to the computer and/or one ormore circuits and configured for projecting the first output images ofthe left output on the projection screen; a right perspective firstprojector coupled to the computer and/or one or more circuits andconfigured for projecting the first output images of the right output onthe projection screen; a left perspective second projector coupled tothe computer and/or one or more circuits and configured for projectingthe second output images of the left output the projection screen; and aright perspective second projector coupled to the computer and/or one ormore circuits and configured for projecting the second output images ofthe right output on the projection screen.

In the twelfth aspect and/or thirteenth aspect the left perspectivefirst projector may comprise a left polarization filter for polarizinglight projected by the left perspective first projector and the rightperspective first projector may comprise a right polarization filter forpolarizing light projected by the right perspective first projector.

The left polarization filter and the right polarization filter may haveorthogonal or approximately orthogonal polarization directions. The leftpolarization filter and the right polarization filter may have oppositecircular polarization directions. The projection screen may be beingnon-depolarizing. The systems according the twelfth aspect and/orthirteenth aspect may further comprise a temporal varying polarizationunit.

BRIEF DESCRIPTION OF THE FIGURES

A multitude of embodiments of the different aspects of the presentinvention are depicted in the figures, where:

FIG. 1 illustrates an example of the prior art,

FIG. 2 illustrates a preferred embodiment of the present invention,

FIG. 3 illustrates details of the preferred embodiment,

FIGS. 4-7 illustrate different pixel values generated in the preferredembodiment.

FIGS. 8-9 illustrate examples of different outputs of the preferredembodiment,

FIGS. 10-12 illustrate alternative embodiments of the present invention,

FIG. 13 illustrates an immersive stereoscopic projection configuration,

FIGS. 14-17 illustrate a preferred embodiment of a projection systemaccording to the present invention,

FIG. 18 illustrates an alternative embodiment of the present invention,and

FIG. 19 illustrates the processing and output of the alternativeembodiment described in relation to FIG. 18.

DESCRIPTION OF THE INVENTION

The present invention is described below in terms of exemplaryconfigurations but is not intended to be regarded as limited to those.For the sake of explanation, greyscale projection systems are used todescribe the present invention, whereas the configurations described mayas well be applied to each of the colour planes of a tri-stimulus (forexample RGB) colour projection system, and, using standard colour spaceconversion techniques, further be used for projection systems usingother colour spaces (for example YPbPr). Further, colour correctioncircuits for adapting for example hue adjustment, black points and whitepoints etc. between source image signals and projectors may obviously beincluded. Still, image projection systems are used in severaldescriptions, whereas the described configurations may as well operateon a sequence of still images constituting a moving image. Monoscopicprojection systems are used in the description, but the invention may aswell apply to a set of projection systems used for stereoscopicapplications or to active stereoscopic projectors with separate left eyeand right eye inputs or with double frame rate inputs. Pixel values aredescribed as being in the range from 0 to 1, whereas in practicalimplementations other ranges will likely be chosen. Operations aredescribed as being performed by separate circuits, whereas in practicalimplementation they will likely be implemented as software algorithms,lookup tables etc. in computer memory or graphics card memory. Furthermodifications, additions and alternative configurations obvious to aperson skilled in the art are intended to be included in the scope ofthe invention.

FIG. 1 shows a schematic view of a configuration of prior art, atraditional double stacking comprising essentially identical projectors,a first projector 1 and a second projector 2, each projecting an imageonto a projection surface 3 and each having a decoding gamma functioncorresponding to the encoding gamma of an image generator 4, which isoutputting a source image signal comprising an array of pixel values.The connecting lines in the schematic view illustrate image signalpaths. The output of the image generator is supplied to the input of thefirst projector 1 and to the input of a warping circuit 5. The output ofthe warping circuit 5 is supplied to the input of the second projector2. The warping circuit 5 performs a geometrical correction of the imageprojected by the second projector 2 to align it with the image projectedby projector 1 and compensate for mechanical misalignment betweenprojected images. Repeated re-calibrations may be needed to compensatefor movements in mechanical and optical parts due to thermal variationsetc.

FIG. 2 shows a schematic view of a first embodiment of the invention. Tothe configuration of FIG. 1 has been added an image splitting functioncomprising a gamma decoding circuit 6, a first gamma encoding circuit 7,a second gamma encoding circuit 8, an image buffer 9, a lightening imagelimiter 10, a first image subtraction circuit 11, a darkening imagelimiter 12, a second image subtraction circuit 13, a first constrainedsmoothing filter 14, a second constrained smoothing filter 15, an imageinversion circuit 16, a first image division circuit 101 and a secondimage division circuit 102, all connected as shown in the figure.

The gamma decoding circuit 6 is matched to the encoding gamma of theimage generator 4, the first gamma encoding circuit 7 is matched to thedecoding gamma of the second projector 2 and the second gamma encodingcircuit 8 is matched to the decoding gamma of the first projector 1.Thus, all operations in the circuit between the output of gamma decodingcircuit 6, the first gamma encoding circuit 7 and the second gammaencoding circuit 8 are performed at a gamma of unity, meaning that pixelvalues represent linear intensities, and the resulting superimposedillumination intensity in a point of the projection surface 3 is afunction of the sum of the corresponding pixel values in the imagesbeing input to the first gamma encoding circuit 7 and to the secondgamma encoding circuit 8.

The image buffer 9 stores a threshold image T which holds for each pixelvalue a representation of the fraction of illumination intensity whichthe first projector 1 is contributing to the corresponding position onthe projection surface 3 when both projectors are supplied uniform,maximum intensity images to their inputs. Since in this embodiment thefirst projector 1 and the second projector 2 are essentially identical,the first projector 1 contributes half the illumination intensity in allpositions, and all pixel values in T are 0.5. In an alternativeconfiguration of this embodiment, the projectors are not identical buthave different spatial distribution of their maximum illuminationintensities; hence T is an image having pixels with varying valuesbetween 0 and 1.

The content T of the image buffer 9 and the output of the gamma decodingcircuit 6 are supplied to the lightening limiter 10. The lighteningimage limiter 10 calculates an image that in every pixel position is thehigher of the two inputs and it outputs the result to the first imagesubtraction circuit 11, which subtracts T and supplies the result to alower bound image input LB of the constrained smoothing filter 14. Thepixel values of this image represents the amount of intensity that thefirst projector 1 is not capable of reproducing alone, hence the minimumintensity the second projector 2 should contribute in the correspondingpixel position.

The content T of the image buffer 9 is supplied to the image inversioncircuit 16 and the output of the image inversion circuit 16 is suppliedto the darkening image limiter 12. Further, the output of the gammadecoding circuit 6 is supplied to the darkening image limiter 12. Thedarkening image limiter 12 calculates an image that in every pixelposition is the lower of the two inputs and outputs the result to anupper bound image input UB of the constrained smoothing filter 14. Thisimage represents the maximum intensity the second projector 2 shouldcontribute, i.e. the desired resulting pixel intensities limited by themaximum intensity the second projector is able to contribute in thecorresponding pixel position.

The first constrained smoothing filter 14 calculates a generally smooth,blurry output image with only few high frequency components and wherethe output image is essentially constrained in any pixel position tohave a pixel value in the range from the corresponding pixel value inthe lower bound image LB and the corresponding pixel value in the upperbound image. FIG. 3 shows a process flowchart of an exemplaryconfiguration of the constrained smoothing filter 14. The constrainedsmoothing filter 14 performs a greyscale dilation operation with adilation radius r1 on the lower bound input image LB followed by a bluroperation with a blur radius r1′ smaller than or equal to r1 on theresult of the greyscale dilation operation, followed by a darkeningimage limiting operation with the upper bound input image UB on theresult of the blur operation, limiting pixel values in the result of theblur operation to be smaller than or equal to the corresponding pixelvalues in the upper bound input image UB and the result of the darkeningimage limiting operation is the output of the first constrainedsmoothing filter. Alternatively, the darkening image limiting operationmay be omitted and the result of the blur operation may be the output ofthe first constrained smoothing filter. The dilation radius r1 may be 4pixels and the blur radius r1′ may be equal to r1. Alternatively, thedilation radius r1 may be 1/300^(th) of the width of the lower boundinput image LB and the blur radius r1′ may be equal to r1. The bluroperation may be a Gaussian blur operation which may have a standarddeviation of ⅓*r1′ or the blur operation may be a mean filteringoperation. In alternative configurations, the first constrainedsmoothing filter 14 may comprise a spline based or membrane basedenvelope filter or a glow effect filter.

The output of the first constrained smoothing filter 14 is supplied to alower bound input of a second constrained smoothing filter 15 and theoutput of the image inversion circuit 16 is supplied to an upper boundinput of the second constrained smoothing filter 15. The secondconstrained smoothing filter 15 may perform an operation similar to thatof the first constrained smoothing filter 14 with a dilation radius r2and a blur radius r2′. The dilation radius r2 may be 2 pixels and theblur radius r2′ may be equal to r2. Alternatively the dilation radius r2may be 1/600^(th) of the width of the lower bound input image of thesecond constrained smoothing filter 15 and the blur radius r2′ may beequal to r2. In an alternative configuration the second constrainedsmoothing filter 15 may be substituted by a blur filter. The dilationradius r2 of the second constrained smoothing filter 15 may beadjustable and the blur radius r2′ may be set to follow r2 whenadjusted. It is noted that when r2=0 and r2′=0, the output of the secondconstrained smoothening filter 15 is equal to the lower bound input,i.e. equal to the output of the first constrained smoothing filter 14.

The output of the gamma decoding circuit 6 and the output of the secondconstrained smoothing filter 15 are supplied to an image subtractioncircuit 13 which calculates an image by subtracting the output of thesecond constrained smoothing filter 15 from the output of the gammadecoding circuit 6. The result of the subtraction is supplied to a firstinput of the first image division circuit 101. The output image T fromthe image buffer 9 is supplied to a second input of the first imagedivision circuit 101. The first image division circuit 101 divides thefirst input by the second input and the result of the division issupplied to the input of the second gamma encoding circuit 8. Hence, thefirst image division circuit 101 scales pixel values in the output imageof the second image subtraction circuit 13, which will be in the rangefrom 0 to the corresponding pixel values of T, by dividing with thepixel values in T, so the resulting output pixel values are scaled to bein the range 0 to 1.

The output image of the second constrained smoothing filter 15 isfurther supplied to a first input of the second image division circuit102 and the output of the image inversion circuit 16 is supplied to asecond input of the second image division circuit 102. The second imagedivision circuit 102 divides the first input by the second input and theresult of the division is supplied to the input of the first gammaencoding circuit 7. Hence, the second image division circuit 102 scalespixel values in the output image of the second constrained smoothingfilter 15, which will be in the range from 0 to the inverse of thecorresponding pixel values of T, by dividing with the inverse of thepixel values in T, so the resulting output pixel values are scaled to bein the range 0 to 1.

The output of the first gamma encoding circuit 7 is supplied to theinput of the warping circuit 5 and the output of the warping circuit 5is supplied to the input of the second projector 2. The output of thesecond gamma encoding circuit 8 is supplied to the input of the firstprojector 1.

In an alternative, simplified configuration of the first embodiment, thedarkening image limiter 12 may be omitted and a uniform, maximumintensity image may be supplied to the upper bound input of the firstconstrained smoothing filter 14.

FIG. 4 shows graphs of values in an example section of a row of pixelsat different stages of the processing, the first graph in FIG. 4 showsthe output of the gamma decoding circuit 6, the second graph shows theoutput of the darkening limiter 12 and the third graph shows the outputof the first image subtraction circuit 11.

FIG. 5 shows three graphs of values in the example section of a row ofpixels at different stages of an operation of the constrained smoothingfilter 14 with a dilation radius r1 of 3 pixels and a blur radius r1′essentially equal to r1. In the first graph in FIG. 5 the result of thedilation operation is indicated as a black line with the lower boundinput indicated in dark gray and the upper bound input indicated inlight gray. The second graph shows in a similar manner the result of theblur operation and the third graph shows the result of the darkeningoperation.

FIG. 6 shows 3 example graphs of the values in a row of pixels, thefirst graph in FIG. 6 shows the output of the second constrainedsmoothing filter 15 when r1=3 pixels and r2=0 and r1′ is essentiallyequal to r1 and r2′ is essentially equal to r2. The second graph showsthe output of the image subtraction circuit 13 and the third graph showssummed values of the output of the second constrained smoothing filter15 and the image subtraction circuit 13, which summed values, as notedabove, trans-late directly to the resulting illumination intensity inthe corresponding row of pixels on the projection surface 3 whenalignment of the projected images is essentially perfect, because theoperations are performed in a gamma of unity. When r2=0 as in thisexample, summation of the input images to the gamma encoding circuits isequal to the output of the gamma decoding circuit 6, which is the gammadecoded source image, hence, with perfect alignment of the projectedimages, the resulting image on the projection surface 3 correspondsessentially perfectly to the output of the image generator 4, acondition that can be referred to as a “perfect reconstruction”. In analternative configuration of this embodiment working in the “perfectreconstruction” condition only, the second constrained smoothing filter15 may be omitted.

As the first graph in FIG. 6 shows, the amount of high spatialfrequencies in the output of the second constrained smoothing filter 15is significantly less than in the output image of the gamma decodingcircuit 6, resulting in a generally smoother, blurred image beingprojected by the second projector 2 than in a traditional doublestacking configuration.

A first advantage of the invention is that the smoother image of thesecond projector 2 reduces the visible artefacts introduced by a smallermisalignment of the projected images. In many cases, a misalignment of afull pixel or more is not noticeable, which in a traditional doublestacking configuration would have introduced highly visible artefacts.

However, as can be seen on the first graph in FIG. 6, the output of thesecond constrained smoothing filter 15 is not completely eliminated highfrequency components. At high contrast edges in the source image wherethe contrast is close to or above the contrast reproduction capabilityof the first projector 1, the upper bound and lower bound inputs to thefirst constrained smoothing filter 14 get so close, so it may not alwaysbe possible to create a smooth “curve” (or rather: surface) betweenthem, and these areas of the projected image will be the most sensitiveto misalignment. Setting r2 to a value higher than 0 will enforce asmoothing also in these areas, reducing spatial frequency componentsfurther and increase the misalignment tolerance. The cost of thisincreased misalignment tolerance is losing the ability of achieving“perfect reconstruction” and introducing small artefacts even at perfectalignment of the projected images, in the form of faint haloes aroundedges in the source image with a contrast higher than the firstprojector 1 is capable of reproducing. Hence, adjusting r2 defines acompromise between “perfect reconstruction” and “high misalignmenttolerance”.

FIG. 7 is equivalent to FIG. 6, except that the dilation radius r2 is 2pixels here and the blur radius r2′ is essentially equal to r2. Thedilation radius r1 is still 3 pixels and the blur radius r1′ is stillessentially equal to r1. The faint halo artefact is visible in thesummed graph at the bottom just to the left of the highest peak.Fortunately, these artefacts may be unrecognizable for the Human VisualSystem in a projected image due to lateral inhibition in the neuralresponse system on the retina (lateral masking), when r2 is below alimit determined by the overall projection system on-screen contrast,hence theoretical “perfect reconstruction” is not necessarily needed.Determining a good value for r2 for a given type of projection systemmay be performed by having a critical group of observers located in thefront rows look at a test pattern containing maximum contrast edges andswitch between random values of r2 and ask the group members to rate theimages in terms of edge sharpness and then selecting the value of r2where nobody notices the reduction of edge sharpness. It is noted thatthe reason for selecting the second constrained smoothing filter 15 alsofor the second filtering pass, as opposed to for example selecting astandard lowpass filter, is that this configuration preservesillumination intensity in small areas of highlights like reflections inwater or leafs, which may be important visual clues that are not subjectto suppression by lateral inhibition.

FIG. 8 shows printed images of an output of the second constrainedsmoothing filter 15 together with the output of the image subtractioncircuit 13 and a simulation of the resulting projected overlaid imagecalculated by adding the output of the second constrained smoothingfilter 15 and the output of the image subtraction circuit 13. (Theimages have here been applied a gamma so they are watchable on print).

FIG. 9 shows similar simulations of an enlarged section of an imageprojected with a 2 pixel misalignment. The upper image is a simulationof a projection with traditional double stacking and the lower image isa simulation of a projection with the first embodiment of the invention.

A second advantage of the invention is that the output image of thesecond constrained smoothing filter 15 will generally not be watchableand not hold enough detail information to be manipulated into awatchable image without additional information being supplied, meaning,that in copy-protected projection systems, where signal paths and imagestorages are subject to encryption and physical anti-tamperingrequirements, the whole signal path from the output of the secondconstrained smoothing filter 15 including the warping circuit 5 and thesecond projector 2 may not need to be encrypted or physically secured.FIG. 10 shows an example of including the first embodiment in a digitalcinema server. An anti-tampering protective housing 18 encompasses theindicated components. The output of the second gamma encoding circuit 8is supplied to an encryption circuit 17 and the first projector 1 is adigital cinema projector capable of decrypting the input image signal.FIG. 11 shows an example of including the first embodiment in a digitalcinema projector. The image generator 4 may be a digital cinema serveroutputting an encrypted image signal, a decryption circuit 19 decryptsthe signal and the anti-tampering housing 18 encompasses the indicatedcomponents. FIG. 12 shows an example of the first embodiment included ina stand-alone unit with an image decryption circuit 18 decrypting theencrypted output of the image generator 4 which may be a digital cinemaserver and an image encryption circuit 17 encrypting the image signaland outputting the encrypted signal to a digital cinema server capableof decrypting the image signal and the anti-tampering housingencompassing the indicated components. In the configurations of FIGS. 9,10 and 11, the first gamma encoding circuit 7, the warping circuit 5 andthe second projector 2 are outside the anti-tampering housing andprocess unencrypted signals, making the practical implementationrelatively uncomplicated.

In an alternative configuration of the first embodiment, a resamplingcircuit may be included, which resamples the output image from the firstgamma encoding circuit 7 to a lower spatial resolution and supplies theresulting resampled image to the warping circuit 5 and where the warpingcircuit and the second projector 2 have lower spatial resolution thanthe first projector 1. Since the output of the first gamma encodingcircuit 7 contains little high frequency components, this may have onlylittle or no effect on the resulting image quality.

Hence, a third advantage of the invention is that upgrade costs may bereduced and investments in existing equipment protected, for example ina theatre with a single 2K projector wishing to upgrade to 4K andincreased brightness. In general, the relaxed requirements to the secondprojector 2 opens up possibilities for asymmetric configurations wherethe second projector 2 may be a completely different projection systemthan the first projector 1, having limitations that would not make ituseful for traditional double stacking but are less significant in aconfiguration of the first embodiment, like lower resolution, slightlyvisible blending edges or brightness differences of a tiled system, notsupporting encryption etc., but having other relevant advantages, suchas good black level, being already installed or being optimised to servespecialized applications when not used as part of the first embodiment,such as conference presentations, planetarium star field projection etc.

In yet an alternative configuration of the first embodiment, an imageerosion circuit is inserted between the output of the first constrainedsmoothing filter 14 and the lower bound input of the second constrainedsmoothing filter 15, where said image erosion circuit performs agreyscale erosion operation on the image signal received from the firstconstrained smoothing circuit 14. The radius R3 of the greyscale erosionoperation may be 0.5 pixel or 1 pixel. This configuration presents theadvantage that errors in actual on-screen pixel intensities due tomisalignment may be shifted into brighter regions, where the same linearintensities will be less noticeable to the human eye due to thenon-linear nature of the human visual system.

In yet an alternative configuration of the first embodiment, a colourcorrection circuit is inserted between the output of the first imagesubtraction circuit 11 and the lower bound input of the firstconstrained smoothing filter 14. Said colour correction circuit isfurther connected to the output of the gamma decoding circuit 6 and itadds to the pixel values in the image received from the first imagesubtraction circuit 11 in a way so that the pixels in the output to thefirst constrained smoothing filter 14 have essentially the same hue asthe corresponding pixels in the image signal received from the gammadecoding circuit 6. This operation may be performed by, for each pixelcalculating a constant K=Max(R11/R6, G11/G6, B11/B6), where (R6,G6,B6)is the pixel colour value of the output of the gamma decoding circuit 6and (R11,G11,B11) is the pixel colour value of the output of the firstimage subtraction circuit 11 and where Max(x,y,z) denotes a functionreturning the highest of the values x, y and z, and by calculating theoutput pixel colour values R′=K*R6, G′=K*G6 and B′=K*B6, and outputting(R′,G′,B′) to the lower bound input of the first constrained smoothingfilter 14. In this configuration, pixel hues in the images projectedfrom both projectors will be the same, which may in some images reducethe visibility of misalignment artefacts further.

In yet an alternative configuration of the first embodiment, the outputsignal from the first gamma encoding circuit 7 or from the resamplingcircuit is recorded on a first medium and the output of the second gammaencoding circuit 8 is encrypted and recorded on a second medium, and thefirst medium and the second medium are played back synchronously withthe output of the first recording medium being supplied to the warpingcircuit 5 which is calibrated for alignment of the images and suppliesthe warped output to the second projector 2 and the output of the secondmedium being supplied to projector 1.

A fourth advantage of the invention is that it may reduce bandingartefacts introduced by a traditional double stacking configuration,because it may have higher dynamic contrast resolution compared to thatof a traditional double stacking system, since more different resultingintensities on said projection surface 3 is possible. In a traditionaldouble stacking configuration where each projector has discreteintensity steps matched to the Just Noticeable Differences of the HumanVisual System, the resulting overlaid image on the projection surface 3may have discrete intensity steps exceeding the Just NoticeableDifferences, which may result in visible banding.

A fifth advantage of the invention is that an automatic re-alignmentsystem based on a digital image capturing system taking pictures ofresulting superimposed image projected on the projection surface 3 mayseparate a captured image into components originating from eachprojector and perform recalibration of the warping circuit without theneed for iterations over a sequence of frames in a public presentationor using special iterating training sequences. For example, a highfrequency filtering of a captured image may create an image that isrelated only to the image being projected by the first projector 1making it possible to do feature matching or tracking, identify a firstset of misalignment vectors from the captured image with respect to theinput image to the first projector 1 and warp the captured image so itis aligned with the first projector 1, and then subtract a gamma decodedversion of the image being input to the first projector 1 from a gammacorrected and gain-corrected version of the captured image, resulting inan image that is related only to the image being projected by the secondprojector 2, so feature matching or tracking is possible and a secondset of misalignment vectors between the captured image and the imagebeing projected by the second projector 2 can be calculated, and fromthe first and second set of misalignment vectors calculate a third setof misalignment vectors, which is the misalignment vectors between theimage being projected by the first projector 1 and the image beingprojected by the second projector 2 and from the third set ofmisalignment vectors perform a re-calibration of the warping circuit 5.Alternatively, in an RGB projection system, a single alignment image maybe constructed which in one colour plane contains a geometric pattern,for example a grid, which has only pixel values above the values in thethreshold image T and where another colour plane contains the samegeometric pattern but with pixel values below the values in thethreshold image T, thus for each pixel position it is possible to obtainrelative misalignment vectors between the projectors and perform are-calibration of the warping circuit 5.

Additionally, the first embodiment may be switchable to a singleprojector mode, in which one of the projectors is simply being suppliedthe source image. This single projector mode may act as fall-backoperation in case of a projector failure and may be activatedautomatically by a detection system capable of detecting a projectorfailure, where the detection circuit may be an integrated part of theprojector or where the detection circuit may be based on a digital imagecapture system taking pictures of the resulting superimposed image beingprojected on the projection surface 3, resulting in a degree ofredundancy, where, for example in the case that a lamp blows, the systemwill continue to project correct images albeit with less brightness.

FIG. 18 shows yet an alternative configuration of the first embodiment,supporting an especially advantageous re-alignment procedure, where animage buffer 103 holding an alignment pattern, an image addition circuit104 and a darkening limiter 105 are added. The output of the imagebuffer 103 is supplied to one input of the image addition circuit 104and the output of the constrained smoothing filter 15 is supplied toanother input the image addition circuit 104, and the output of theimage addition circuit 104 is supplied to one input of the darkeninglimiter 105 and the output of the gamma correction circuit 6 is suppliedto another input of the darkening limiter 105 and the output of thedarkening limiter is supplied to one input of the image division circuit102 and to one input of the image subtraction circuit 13, as shown inthe figure. The output of the image buffer 103 may be switchable betweena black picture and the alignment pattern, so the alignment pattern caneffectively be switched off, when alignment detection is not requested.The effects of these added circuit elements on the projected images arethat the image projected by projector 2 will be added a constrainedalignment pattern, which is the output of the image buffer 103 beingconstrained, so that the result of the addition in each pixel positionis still equal to or lower than the intensities of the correspondingpixel values in the source image, and the image projected by projector 1will be subtracted the constrained alignment image, so when the twoimages are superimposed on the projection surface 3 with perfectalignment, the alignment pattern will be cancelled out and becomeinvisible, so only the source image is visible. However, when amisalignment is introduced, the alignment pattern becomes visible aspattern sections of lower and higher intensities than the surroundingpixels. This enables easy and precise visual detection of any presentmisalignment. The position of lower and higher intensities indicates inwhich direction the misalignment is oriented. For example, if a sectionof an alignment pattern is visible as lighter pixel values compared tothe surroundings, i.e. a lighter pattern imprint, and the same sectionof the alignment pattern is visible as darker pixel values compared tothe surroundings, i.e. a darker pattern imprint, and the dark imprint islocated to the right and below the light imprint, this indicates thatprojector 1 is displaced to the right and towards the lower edgerelative to the position in which perfect alignment occurs. In this way,detection of misalignment can be executed during operation of theprojection system, and even correction may be performed by adjusting thewarping circuit 5. The alignment pattern may be designed, so it is notvery noticeable to a general audience, though still useful for aprojectionist, for example by comprising small graphic elements withregular spacing.

The alignment pattern may be a grid, a mesh or any regular or irregularpattern of elements which may be dots, cross hairs or other graphicelements and it may contain barcodes, semacodes or other identifiers.

FIG. 19 shows example signals of the configuration of FIG. 18, where thefirst image is the output the darkening limiter 105 with the addedalignment pattern visible, the second image is the output of imagesubtraction circuit 13 with the subtracted alignment pattern visible,the third image is the resulting superimposed image on the projectionsurface 3 with perfect alignment and the fourth image is an example of aresulting superimposed image on the projection surface 3 whenmisalignment is present.

In a colour image projection system comprising multiple configurationsof the first embodiment each projecting a colour plane of the image, afirst colour plane may be projected with an alignment pattern by theconfiguration shown in FIG. 18 and the other colour planes may beprojected without alignment patterns. When the colour planes areprojected by the same physical projectors, the mechanical misalignmentof projectors and projection optics will affect the colour planesessentially identical, so the misalignment information observed from thefirst colour plane can be used to detect and correct the misalignment ofall colour planes. This will further reduce the visibility of thealignment pattern to a general audience, especially if the colour planewith the alignment pattern is the blue colour plane, whereas theprojectionist can observe the image through an optical filter havingessentially the same colour as the colour plane with alignment image,thereby increasing the visibility of the alignment image to theprojectionist.

Alternatively to having a projectionist observing the image manually, acamera may record the image on the projection surface 3 and an imageprocessing system may detect and correct misalignment. The imageprocessing system may perform feature matching or feature tracking, forexample scale invariant feature tracking, to perform recognition of thealignment pattern or alignment pattern sections. Further, the camera mayhave a long exposure time, so that several different projected images,for example subsequent frames of a moving picture, are integrated in theimage capturing element over one exposure, thereby blurring allnon-static picture elements, but preserving the static alignment patternfor easier recognition of alignment pattern or alignment patternsections. For example, the alignment pattern or alignment patternsections may be separated from the integrated and blurred image by ahigh pass filtering. A sequence of images to be projected may bepre-processed, to increase the blurring of other elements than thealignment pattern when later integrated in the camera's image capturingelement, for example a slow, cyclic motion may be introduced to staticscenes of a sequence of a moving picture, or one of the colour planes,for example the blue colour plane, may be blurred in one or more or allof the frames of the moving picture.

In a colour image projection system comprising multiple configurationsof the first embodiment each projecting a colour plane of the image, anadditional colour correction circuit may be comprised, which adds to thepixel values in the colour channels of the outputs of the firstconstrained smoothing filters 14 in a way so that the hue of the pixelsin the output of the first constrained smoothing filters 14 areessentially identical to the hues of the corresponding pixels in theoutput of the gamma decoding circuit 6. The additional colour correctioncircuit may perform an operation, where it for each pixel calculates afraction value, which is the pixel value of the output of the firstconstrained smoothing filter 14 divided by the corresponding pixel valueof the output of the gamma decoding circuit 6, then the additionalcolour correaction circuit identifies the greatest of the fractionvalues for each of the colour planes, i.e. for each of the multipleconfigurations of the first embodiments, and for each of the colourplanes, a new pixel value is calculated by multiplying the output of thegamma decoding circuit 6 with the fraction value for the colour plane,and the resulting pixel value is supplied to the input of the secondconstrained smoothing filter 15. The advantage of this colour projectionsystem is that the hues projected from the first projector 1 and fromthe second projector 2 will for each pixel be essentially identical,which may further decrease visible artefacts resulting frommisalignment.

In a specially advantageous configuration, a 3D system is comprising twoimage processing circuits according to the first embodiment, a firstimage processing circuit according to the first embodiment beingsupplied a left perspective image of a 3D image and a second imageprocessing circuit according to the first embodiment being supplied aleft perspective image of said 3D image and three projectors, twostationary polarization filters, a temporal varying polarization unit,such as the RealD ZScreen or the RealD XL polarizing beam splitterarrangement with ZScreens, a non-depolarizing projection screen andeyewear with polarizers. A first projector is supplied the output of thesecond gamma encoding circuit 8 of said first image processing systemand has a first polarization filter inserted in the optical path betweenthe light source of said first projector and said projection screen, asecond projector is supplied the output of the second gamma encodingcircuit 8 of said second image processing system and has a secondpolarization filter inserted in the optical path between the lightsource of said second projector and said projection screen, said firstpolarization filter and said second polarization filter havingessentially orthogonal polarization directions or opposite circularpolarization direction, and where a third projector is projectingalternately the output of the first gamma encoding circuit or theresampling circuit of said first image processing system and the outputof the first gamma encoding circuit or the resampling circuit of saidsecond image processing system. In other words, two separate projectionsystems, one for a left eye image and one for a right eye image, useeach one projector for the high frequency image and share a timemultiplexed projector for the low frequency image.

The advantage of this configuration is that the third projector projectsalternately the overlay images of the left and right perspective imagesthat have low amounts of high frequency components, therefore therequirements to the performance of this projector in terms of resolutionare relaxed, again allowing to optimize the projector for brightness onthe cost of some resolution or image sharpness, for example utilizing apolarizing beam splitter with image combiner, such as for example theRealD XL adapter, which essentially doubles the light output of theprojector, but at the cost of limiting the maximum obtainable resolutionin practical implementations. This way, the same amount of lightreaching the screen as with four projectors can be achieved using justthree projectors. For example, a 3D projection system comprising threeprojectors with a 7 KW Xenon lamp each could result in the samebrightness as that of a system comprising four projectors with 7 KWlamps each, which could be adequate for illuminating 3D giant screens.Such a system could rival existing filmbased 3D projection systems forgiant screens in both image resolution, brightness, image stability,contrast, dynamic range and frame rate.

FIG. 13 shows an immersive, stereoscopic projection configuration with atotal of four overlaid projectors, a first left projector 121, a secondleft projector 122, a first right projector 123 and a second rightprojector 124, where the first left projector 121 and the second leftprojector 122 are parts of a configuration according to the firstembodiment and are projecting a left view of a stereoscopic image andwhere the first right projector 123 and the second right projector 124are parts of a configuration according to the first embodiment and areprojecting a right view in an immersive giant screen theatre where theprojection surface 3 may be a domed screen or a big flat screen locatedclose to the audience so a large portion of the field of view of themembers of the audience located in the theatre seats 125 is filled withimage and where the audience members are wearing stereoscopic eyewear.The projectors may be located off-axis close to the edge of the domedscreen and may comprise wide angle or fisheye projection optics. Theprojection optics may be constructed so that pixel density is higher inan area, a “sweet spot”, in front of the audience, as is well known inthe art of immersive projection. The projection optics may furthercomprise anamorphic adaptors, which stretch the image in the verticaldirection to fill a larger area of the dome. Additional warping circuitsmay be comprised, which performs a geometrical correction of the lefteye source image and the right eye source image. The warping circuitsmay operate individually on each of the colour planes of the sourceimages so they can be calibrated to further compensate for chromaticaberration in the projection optics. Alternatively to including imagesplitting circuits according to the first embodiment in theconfiguration, a playback system may be included, capable ofsynchronously reproducing previously recorded outputs from an imagesplitting circuit according to the first embodiment stored on at leastone storage medium and supplying the reproduced outputs to theprojectors. The storage medium may comprise at least one hard diskcontaining a first set of assets comprising a first signal for the firstleft projector 1, where the first signal is the recorded output of thesecond gamma encoding circuit 8 when the left source image was suppliedto the input of the gamma decoding circuit 6 and a second signal for thefirst right projector 1, where the second signal is the recorded outputof the second gamma encoding circuit 8 when the right source image wassupplied to the input of the gamma decoding circuit 6, and furthercontaining a second set of assets comprising a third signal for thesecond left projector, where the third signal is the recorded output ofthe first gamma encoding circuit 7 when the left source image wassupplied to the input of the gamma decoding circuit 6, and a fourthsignal for the second right projector, where the fourth signal is therecorded output of the first gamma encoding circuit 7 when the rightsource image was supplied to the input of the gamma decoding circuit 6.The first set of assets may be stored on the hard disk in the format ofa stereoscopic Digital Cinema Package and the second set of assets maybe stored on the hard disk in the format of a stereoscopic DigitalCinema Package. The first set of assets may be stored in an encryptedform and the playback system may be able to supply an encrypted signalto the input of the first left projector and an encrypted signal to theinput of the first right projector. Further, a first warping circuit maybe comprised located in the signal path from the playback system to thesecond left projector and a second warping circuit may be comprisedlocated in the signal path from the playback system and the second rightprojector where the first warping circuit and the second warping circuitare calibrated for alignment of the images.

The projectors in the configuration of FIG. 12 may use spectralseparation for separating the left and right eye views, where members ofthe audience wear eyewear with dichroic spectral separation filters andwhere the projectors comprise dichroic spectral separation filters. Theseparation filters of the first left projector 121 and the second leftprojector 122 may be essentially identical and the left eye separationfilter in the eyewear may be matched to the separation filters of thefirst left projector 121 and the second left projector 122 and theseparation filters of the first right projector 123 and the second rightprojector 124 may be essentially identical and the right eye separationfilter in the eyewear may be matched to the separation filters of thefirst right projector 123 and the second right projector 124. Spectralseparation stereoscopic projection has the advantage of not requiring aspecial projection surface which is attractive in many immersive cinemaapplications, and it has very good image quality and stereoscopicreproduction in a central part of the field of vision, but it has thedisadvantage of introducing artefacts outside of the central part of thefield of vision, because the filters in the eyewear differ from theirnominal performance for incident light with angles not normal(perpendicular) to the filters, a phenomenon which is inherent in thenature of dichroic filters. For these reasons, an improved system forspectral separation stereoscopic projection shall be proposed below.

FIG. 14 shows an example of prior art. A lamp 20 in a first projectoremits light into an integrating rod 21 which creates a uniformillumination at the output end. A first projector filter 23, being adichroic spectral separation filter resting on a glass substrate 22, islocated adjacent to the output of the integrating rod 21, essentially ina focal plane of the illumination system 24, so an image of the firstprojector filter 23 is essentially focused on the spatial lightmodulator chips 25 of the projector. A second projector (not shown) isconfigured equivalently but with a second projector filter (not shown),which is mutually exclusive to the first projector filter 23. The firstprojector filter 23 and the second projector filter have mutuallyexclusive pass bands and in between there are spectral ranges calledguard bands where both the first projector filter 23 and the secondprojector filter have little transmittance. The left eye separationfilter in the eyewear may be a dichroic filter having a set of passbands encompassing the pass bands in the first projector filter 23 andthe right eye separation filter in the eyewear may be a dichroic filterhaving a set of pass bands encompassing the pass bands in the secondprojector filter. The separation filters in the eyewear may be slightlycurved to partly compensate for the non-normal (non-perpendicular) angleof incident light from pixels in the peripheral areas of the image asobserved by a member of the audience positioned with her head directedessentially straight forward with her nose towards the screen, becauselight with a non-orthogonal angle of incidence travels a longer distancebetween the dichroic layers of the separation filters, hence is subjectto a filtering where the pass bands have been spectrally shiftedcompared to the filtering of light from pixels in the middle area of theimage with essentially normal (perpendicular) angle of incidence, whichwould otherwise cause the match with the projector filters to be reducedbeyond the tolerances provided by the guard bands in the projectorfilters, giving rise to colour artefacts and artefacts of crosstalkbetween left and right projection systems (“ghosting”) in the peripheralparts of the image. It is normally not practical to use separationfilters that are curved enough to completely compensate for the anglesof incident light from different parts of the image for aestheticreasons regarding the eyewear design and because the distance betweeneyes varies significantly in a population of different ages. Theexperience of the remaining artefacts in the peripheral parts of theimage may be described as having a sheet of slightly coloured,semi-transparent, semi-reflective material with two fuzzy holes in frontof your eyes attached to your head, the holes not completely coveringthe image, resulting in a sense of “tunnel vision”. Therefore, furthermeans to reduce the artefacts in the peripheral parts of the image areusually adopted comprising pre-wavelength shifting the projectorfilters, increasing the width of the guard bands at the cost of reducedbrightness and further comprising reducing the size of the eye openingsin the eyewear limiting the range of possible angles of incident light,thereby introducing a sharp and psychologically better accepted borderof your field of view but obviously at the cost of a restricted field ofview. However, for an immersive cinema application, artefacts in theperipheral field of vision will not be completely eliminated.

FIG. 15 shows an alternative configuration of the system in FIG. 14where the colour and ghosting artefacts in peripheral parts of the imageare compensated by modifying the first projector filter 23 and thesecond projector filter so the spectrally filtered light at the exitpupils of the projectors becomes wavelength shifted as a function of theangle of emission. The first projector filter 23 and the secondprojector filter are curved with essentially identical curves, so thatlight focused on pixels in the peripheral parts of the light modulatorchips traverse longer distances between the dichroic layers than lightfocused on the central parts of the light modulator chips, hence lightfocused on the peripheral parts of the light modulator chips iswavelength shifted with respect to light focused on the central parts ofthe light modulator chips and therefore light emitted from pixels in theperipheral parts of the projected image is wavelength shifted withrespect to light emitted from pixels in the central parts of theprojected image, resulting in a better match of the filtering by theprojector filters and the filtering of the eye filters for pixels in theperipheral parts of the image, and in more pixels in the peripheralparts of the image being filtered by the eye filters so that the passbands of the eye filters encompass the pass bands of the projectorfilters when observed by a member of the audience in a targetobservation position. Other members of the audience located at otherpositions may observe a slightly undercompensated or overcompensatedimage, but still observe a better image than without compensation. Thecurve of the first projector filter 23 and the second projector filtermay be spherical with radii equal to the width of the aperture of theintegrating rod 21. An electronic colour correction is normally appliedto the source image to compensate for a slight hue changes as perceivedby the Human Visual System in the filters, which cannot be avoidedcompletely for manufacturing reasons. This colour correction is normallyspatially uniform over the image area. In the case of using curvedfilter, this colour correction may instead be spatially nonuniform, soas to achieve projected images that are perceived as uniform in hue tothe Human Visual System. Alternatively to comprising curved filters,dichroic filters with varying thickness of the dielectric layers may becomprised.

The experience of watching an image compensated with curved filter ishard to describe, but appears somewhat more pleasing than the“uncompensated experience”. It can be described as enlargening the fuzzyholes in the slightly coloured, semi-transparent, semi-reflective sheetso the full image can be seen through them when your face is orientedforwards towards the screen, but the sheet is now detached from yourhead, though still close, so when you move your head away from thestraight looking forward orientation, the edges of the fuzzy holes enteryour field of vision, like gazing through a pair of holes in thindrapes.

FIG. 16 shows an alternative configuration, where the first projectorfilter 23 and the second projector filter may each have a flat area in acentral region and only have a curved shape in the peripheral areas ofthe image where the tolerance by above the mentioned other means ofreducing the artefacts in peripheral areas of the image do not suffice.The optimal curve of the projector filters is a function of the distancefrom the member of the audience to the screen, the curve of the eyefilters, the focal length of the relay lenses of the illumination systemof the projectors, subjective aesthetic preferences and other factors. Acompromise between the “tunnel vision” and “gazing through a pair ofholes” may be desirable.

FIG. 17 shows yet an alternative configuration equivalent to theconfiguration of FIG. 14, but where a first curved notch filter 27resting on a first glass substrate 26 is added located in front of thefirst projector filter 23 in the left projector and a second curvednotch filter resting on a second glass substrate are addedcorrespondingly in the second projector, and where the notch filtershave notches essentially matching the guard bands, so the width of theguard bands are being widened as a function of the emission angle oflight exiting the exit pupils of the projectors, hence reducingartefacts in the peripheral field of vision and eliminating ghostingartefacts in the central field of vision in the case where the observerturns her head to a large angle that may occur in the configurationsaccording to FIGS. 14, 15 and 16, although at the cost of reducing thebrightness in the peripheral parts of the projected images. The notchfilters may have a flat area in a central part of the image.

The invention is additionally or alternatively characterized by an imageprocessing circuit separating an input image into a first image, beingthe input image clamped to a threshold, and a second image, being theremainder. The second image is smoothened by moving fractions of pixelvalues from the first image to the darker areas around edges, reducingthe content of high-frequency components in the second image, keepingthe sum of the two images identical to the input image. Scaling andgamma corrections are performed at the input and outputs, ensuring thatactual luminance superposition applies to the calculations. With perfectalignment, the projected overlaid image will correspond exactly to theinput image, whilst the second image will have less high frequencycomponents than the first image.

A first advantage is that the system significantly reduces the perceivedartefacts arising from minor misalignment, since the human visual systemis less sensitive to errors in low frequency components than in highfrequencies. Only where there are edges having a contrast higher thanone projector can “drive” alone, the second image will contain highfrequency components. However, the human visual system exhibits a lowerspatial resolution close to edges of contrasts of 150:1 and above, dueto the so called spatial masking effect, so misalignment artefacts athigh contrast edges will also be reduced in visibility. A low-passfiltering of the second image, moderate enough to be invisible due tothe masking effect of the first image's high frequency components, mayhelp masking misalignment artefacts at high contrast edges further.

A second advantage is that a camera-based automatic alignment system canperiodically perform realignment throughout a film projection, based onthe images in the film, with no need for special calibration sequenceruns. Because the projectors do not project identical images, it ispossible to separate the first and the second image from the recordedon-screen image, and from those calculate misalignment information,which in turn may be used for electronic re-alignment by geometriccorrection (warping).

A third advantage is that a single-projector 2K system can be upgradedto increased brightness and 4K resolution, by adding a 4K projector.Since an invisible moderate low pass filtering of the second image ispossible, it results in that it is possible to use a lower resolutionprojector for the second image, maintaining the appearance of the fullhigh resolution of the first projector (only brighter). A fourthadvantage is that the resulting luminance resolution of the system ishigher than that of a single projector, which could be of significanceto high dynamic range projection systems.

The invention is additionally or alternatively characterized by thepoints:

1. An image projection system comprising two image projectors, a firstprojector and a second projector, where said first projector and saidsecond projector project overlaid images onto a projection surface,resulting in a superimposed image, further comprising a first imageprocessing circuit, which separates an input image into two images: afirst projector image being input to said first projector and a secondprojector image being input to said second projector, so that when saidfirst projector is projecting said first projector image and said secondprojector is projecting said second projector image, the overlaid imageformed on the projection surface essentially corresponds to said inputimage, and where the amount of high spatial frequencies is lower in saidsecond projector image than in said first projector image.

2. An image projection system according to point 1, where colourcorrection circuits are added to both of said projector's inputs,calibrated so that the resulting projector transfer functions betweenpixel values and projected colour plane luminances become essentiallylinear and identical, so that the resulting projected colour planeluminances at a point on the display surface is essentially a functionof the sum of the corresponding pixel values of said first projectorimage and the corresponding pixel values of said second projector image,when said corresponding pixel values of said first projector image iswithin the range 0 to B1 and said corresponding pixel values of saidsecond projector image is within the range 0 to B2, where B1 is thepixel value corresponding to the maximum colour plane luminance of saidfirst projector and B2 is the pixel value corresponding to the maximumcolour plane luminance of said second projector, and where thecalculation of said second projector image comprises, for each pixelvalue of essentially all pixels in the input image, calculating thevalue that exceeds B1, and where said first projector image iscalculated by subtracting said second projector image from said inputimage, and where the pixel values of said input image is within therange 0 to B, where B=B1+B2 is the pixel value corresponding to maximumcolour plane luminance of the resulting superimposed image.

3. An image projection system according to point 2, where saidcalculation of said second projector image further comprises a smoothingprocess, adding amounts to pixel values in said second projector imagein a way, so that high frequency components in said second projectorimage are reduced, and where said amounts are limited to be within zeroand the corresponding pixel values in said first projector image.

4. An image projection system according to point 3, where said smoothingprocess comprises adding halos to edges in said second projector image,where the halos extend into the darker side of the edges graduallyfading with increasing distance from the edges.

5. An image projection system according to point 3 or 4, where saidsmoothing process comprises a weighted greyscale dilation applied toeach of the colour planes of said second projector image, where saidweighted greyscale dilation is defined as a greyscale dilation with astructuring element D and where the input pixels are first multiplied bythe elements of a filtering kernel F.

6. An image projection system according to points 1-5, furthercomprising a low-pass filter with a convolution kernel L or othersmoothening filter inserted between said first image processing circuitand said second projector.

7. An image projection system according to points 1-6, where said secondprojector has a lower spatial resolution than said first projector.

8. An image projection system according to points 5-7, where thegreyscale dilation structuring element D is a disc shaped element with aradius of 0.2% of the image width, the filtering kernel F is a distancefunction with a radius of 0.2% of the image width and the convolutionkernel L is a Gaussian kernel with a radius of 0.1% of the image width.

9. An image projection system according to points 1-8, furthercomprising an automatic alignment system comprising at least one cameracapable of recording images of said resulting projected image on saidprojection surface and a second image processing circuit, capable ofisolating a first set of features originating from said first projectorimage in an image recorded by said camera(s) and isolating a second setof features originating from said second projector image in said imagerecorded by said camera(s) and capable of spatially correlating saidfirst set of features and said second set of features to features ofsaid input image and from said correlations calculating spatialmisalignment information, further comprising a third image processingcircuit, capable of geometrically correcting at least one of said firstprojector image and said second projector image, based on saidmisalignment information, so said first projected image and said secondprojected image become geometrically aligned.

10. An image projection system according to point 9, where said secondimage processing circuit comprises a colour correction circuit,producing from said recorded image a conformed recorded image,calibrated so that the transfer function between pixel values and colourplane luminances of the overlaid image on said display surface isessentially identical to said projector transfer functions, and wheresaid second image processing circuit seeks to identify at least onelow-luminance area(s) in which all pixel values of said conformedrecorded image are below a threshold T, where T is less than or equal toB1, and performs a first set of feature matching operations with saidfirst projector image in at least one feature matching area(s) withinsaid low-luminance area(s) resulting in a first set of offset vectors,and where said second image processing circuit can perform a geometricalcorrection of said conformed recorded image based on said first set ofoffset vectors, so that the geometrically corrected, conformed recordedimage is aligned with said input image and where said second imageprocessing circuit subtracts said first projector image from saidgeometrically corrected, conformed recorded image and on the resultingimage performs a second set of feature matching operations in at leastone area(s) with said second projector image resulting in a second setof offset vectors, and where said third image processing circuit iscapable of geometrically correcting at least one of said secondprojector image and said second projector image based on said first setand said second set of offset vectors, so said first projected image andsaid second projected image become essentially geometrically aligned,and where said feature matching operations may be template matchingoperations, scale invariant feature tracking operations or any otherfeature tracking operations known in the art.

11. An image projection system according to points 9 and 10, where saidautomatic alignment system perform repeated cycles during presentationof a moving picture, a live transmission, a still image or othercontent, to reduce geometric misalignment arising during projection.

12. An image projection system according to points 1-11, where more thantwo projectors are projecting overlaid images, said first imageprocessing circuit outputting more than two images, each havingdifferent amounts of spatial frequencies and where said second imageprocessing circuit is capable of isolating features in said recordedimage originating from each of said projectors.

13. An image projection system according to points 1-12, furthercomprising any modifications and configurations included in thetechnical description or evident to a person skilled in the art.

The invention is additionally or alternatively characterized by theadditional points:

1. An image projection system comprising an essentially hemispheric,dome shaped projection surface and at least one image projector locatednear the edge of said domed shaped projection surface, where said imageprojector projects an image onto the inside of said dome shapedprojection surface and where the projected image covers at least 70% ofsaid dome shaped projection surface, comprising a wide angle projectionobjective, a fish-eye projection objective, a wide-angle conversionlens, a wide-angle conversion mirror, an inverse afocal optical systemor a retrofocus optical system or a combination of any of these, furthercomprising a first image processing circuit which performs a geometricalcorrection of an input image and sends a corrected output image to theinput of said projector.

2. An image projection system according to the additional point 1further comprising an anamorphic adaptor comprising at least one prismlocated in the light path between the image forming element and thescreen, where said anamorphic adaptor is stretching said image in onedirection.

3. An image projection system according to additional points 1 or 2,where said first image processing circuit is calibrated, so that saidprojected image essentially has the same geometry as a projected imagefrom a fish-eye projector located essentially at the center of saidhemispheric, dome shaped projection surface, when said input image isbeing input to said fish-eye projector.

4. An image projection system according to additional points 1-3, wheresaid first image processing circuit is able to perform separategeometrical corrections of each of the colorplanes of said input image,and where said first image processing circuit is calibrated so that saidgeometrical corrections compensates for chromatic aberrations in theoptical elements of said image projection system.

5. An image projection system according to additional points 1-4, whereat least one area located in said dome shaped projection surface has ahigher spatial resolution than the average spatial resolution of saidprojected image, and where said input image has a higher spatialresolution than said corrected output image, and where said imageprocessing circuit essentially preserves as much spatial resolution fromsaid input image to said output image as possible.

6. An image projection system according to additional points 1-5,further comprising a second image processing circuit able to calculatefrom said corrected output image a reflection-error image, where saidreflection-error image is an estimate of the total reflected light thatwill be received at each position on the display surface from otherparts of the display surface by scattering, if said input image were tobe projected onto the display surface by said projector, where saidreflection-error image may be calculated based on a set of screenmeasurements and where said reflection-error image may be calculated byradiosity calculations, and where said image processing circuitessentially subtracts said reflection error image from said input image(negative values being set to zero) resulting in a compensated image,which may be sent to the input of said projector.

7. An image projection system according to additional point 6, wherelocal contrast enhancement is applied to areas of said compensatedimage, where full cancellation of reflected light is not achieved by thesubtraction of said reflection-error image.

8. An image projection system according to the additional point 7, wherea remainder-error image is calculated as the difference between saidreflection-error image and the result of a subtraction of saidcompensated image from said corrected output image, and where a contrastenhanced compensated image is calculated from said compensated image bylocal contrast enhancement and where said remainder-error image islow-pass filtered and then used as a key in a keying operation betweensaid compensated image and said contrast enhanced compensated image, andwhere the resulting image of the keying operation is sent to the inputof said projector.

9. An image projection system according to additional points 7 or 8,where said local contrast enhancement is an unsharp mask operation or alocal tone mapping operation.

10. An image projection system according to additional points 1-9,further comprising any modifications and configurations included in thetechnical description or evident to a person skilled in the art.

The invention claimed is:
 1. A method of producing from a received imageand a threshold image a first image and a second image, comprising:determining an upper bound image and a lower bound image based on pixelvalues of each of the received image and the threshold image; producingthe first image and the second image by using a smoothing filter processin combination with the upper bound image, the lower bound image, thereceived image and the threshold image, wherein the first image has adifferent spatial content frequency than the second image based on thesmoothing filter process; projecting the first image onto a surface by afirst projector; and projecting the second image onto the surface by asecond projector, wherein the first image and the second image havepixel values that are within illumination limits of the first projectorand the second projector.
 2. The method of claim 1, wherein the lowerbound image represents pixel values that exceed a illumination intensitylimit of the first projector or the second projector.
 3. The method ofclaim 1, wherein the upper bound image represents maximum pixel valuescontributable by the first projector or the second projector onto thesurface.
 4. The method of claim 1, wherein determining the lower boundimage comprises: i. determining for each corresponding pixel between thereceived image and the threshold image a maximum pixel value; and ii.determining a difference value by subtracting a corresponding thresholdvalue from the maximum pixel value for each pixel, wherein determiningthe upper bound image comprises determining for each corresponding pixelbetween the received image and an inverted threshold image the minimumpixel value.
 5. The method of claim 1, further comprising: balancingillumination between the first projector and the second projector bycontrolling a representation of a fraction of illumination intensity inthe threshold image for each pixel of which the first projector and thesecond projector contributes to the surface.
 6. The method of claim 1,wherein the smoothing filter process comprises: i. receiving the upperbound image and the lower bound image; and ii. performing a dilationoperation followed by a blur operation on each pixel of the lower boundimage.
 7. The method of claim 6, wherein the smoothing filter processfurther comprises determining a minimum of corresponding pixels of afiltered lower bound image and the upper bound image for each pixel. 8.The method of claim 1, further comprising: gamma decoding the receivedimage.
 9. The method of claim 1, further comprising: gamma encoding thefirst image prior to projecting the first image; and gamma encoding thesecond image prior to projecting the second image.
 10. The method ofclaim 1, further comprising: warping at least one of the first image orthe second image prior to projecting the first image and projecting thesecond image.
 11. The method of claim 1, further comprising: balancingintensity of the first image and the second image, wherein the smoothingfilter process comprises a constrained smoothing filter process.
 12. Themethod of claim 1, wherein projecting the second image onto the surfaceby the second projector comprises: causing the second image to besuperimposed and geometrically aligned with the first image projected bythe first projector.
 13. The method of claim 1, wherein producing thefirst image comprises: determining a difference of corresponding pixelsbetween the received image and an image outputted from the smoothingfilter process; and dividing by corresponding pixels of the thresholdimage for each pixel.
 14. The method of claim 13, wherein producing thesecond image comprises dividing an output of the smoothing filterprocess with corresponding pixels of an inverted threshold image foreach pixel.
 15. An image projection system, comprising: image processingcircuitry configured for: generating a first image and a second imagefrom an input image using a constrained smoothing filter process,wherein the first image and second image are different based on theconstrained smoothing filter process; balancing intensity of the firstimage and the second image; and producing a warped image by warping thesecond image; a first projector for projecting the first image; a secondprojector for projecting the warped image; and a screen on which thefirst image projected by the first projector is configured to besuperimposed and geometrically aligned with the warped image projectedby the second projector.
 16. The image projection system of claim 15,wherein the image processing circuitry is configured for: determining anupper bound image and a lower bound image based on pixel values of eachof the input image and a threshold image; generating the first image andthe second image by using the constrained smoothing filter process incombination with the upper bound image and the lower bound image; andcausing the first image and the warped image to have pixel values thatare within illumination limits of the first projector and the secondprojector.
 17. An image projection system, comprising: a first projectorfor projecting a first image; a second projector for projecting a secondimage having different spatial content frequency than the first image tooverlay the first image on a projection surface; and image processingcircuitry adapted for generating the first image and the second imagefrom a received image using at least one smoothing filter in combinationwith an upper bound image and a lower bound image determined based onpixel values of each of the received image and a threshold image. 18.The image projection system of claim 17, wherein the image processingcircuitry is adapted for: generating the first image and the secondimage having pixel values that are within illumination limits of thefirst projector and the second projector; and generating the secondimage having different spatial content frequency than the first imagebased on the at least one smoothing filter.
 19. The method of claim 1,further comprising: limiting a projector illumination intensity demandto within an illumination intensity contribution capability of each ofthe first projector and the second projector using the threshold image.20. The method of claim 19, wherein the threshold image is a fraction ofa total illumination intensity that is contributed by the firstprojector or the second projector, the total illumination intensitybeing an illumination on the surface of a corresponding pixel value ofthe first image and the second image projected on the surface, thefraction representing the illumination intensity that one of the firstprojector or the second projector is able to contribute to thecorresponding pixel value.
 21. The method of claim 20, wherein thethreshold image is a spatial distribution of the fraction of the totalillumination intensity that is contributed by the first projector or thesecond projector.
 22. The method of claim 1, wherein the first imageprojected onto the surface and the second image projected onto thesurface are misaligned by a misalignment amount to produce a visiblemisalignment artefact that is less visible than the first projector andthe second projector projecting the received image with a misalignmentof the misalignment amount.